Fumigants are widely used for the disinfestation, and protection against infestation, that is usually required to protect particulate commodities (such as grain) and other stored products (including durable and perishable foodstuffs or cut flower), porous bulk materials (for example, soil or timber) and spaces (either empty buildings or building containing commodities). An ideal fumigant should be toxic to insects, psocids, mites, nematodes, bacteria, fungi and their spores, viruses and moulds and other pest biota. It should be effective in low concentrations. It should ideally have a low absorption by materials in the fumigated region. It should have a low phytotoxicity to commodities. It should have a low mammalian chronic toxicity and leave either no residue or an inert residue. In addition, the ideal fumigant should present no difficulties as far as safe handling is concerned, and it should not adversely affect the commodity or space that is being fumigated.
No fumigant meets all of these “ideal” criteria. The two fumigants most commonly used in the fumigation of grain, other particulate materials, fruit and timer are phosphine and methyl bromide. However use of methyl bromide is due to be phased out in Australia and other developed countries after 2005. Carbon disulphide was recently proposed as an alternative to these fumigants (WO 93/13659) but is no longer registered for use as a fumigant in New South Wales, Australia. As a result, phosphine is expected to become the only registered fumigant available for farm use in Australia.
Phosphine is the preferred fumigant for grain stored and the like because it is effective against grain pest and leaves little residue (which is essentially a harmless phosphate). However, phosphine is spontaneously combustible when its concentration exceeds a relatively low value, and is unable to kill all stages of insects in a short period when used at acceptable concentrations.
Fumigation with phosphine requires a long (>5 days) exposure in sealed bins at temperatures above 15° C. The many existing farm bins are unsealed, and are therefore unsuitable for effective fumigation, as concentrations cannot be maintained for the time required for total insect control. The over-reliance on phosphine and unsealed bins in Australia has resulted in (1) a higher frequency of resistance, (2) dangerous practices, and (3) grain delivered to grain depots containing live insects and un-reacted aluminium phosphide residues.
Alkyl formates such as ethyl formate and methyl formate have a long history of use as fumigants for stored products. Ethyl formate is currently registered as a fumigant for dried fruit in Australia and is now being investigated as an alternative fumigant for grain stored in unsealed farm bins in Australia. Care must be taken when working with alkyl formatea to keep concentrations in the structure to below the flammable level. This is done by controlling the rate of vaporisation to maintain an effective concentration of the alkyl formate for a sufficient time in the structures by avoiding accumulation of liquid alkyl formate at the bottom of the stored grain structure.
Cyanogenic compounds such as hydrogen cyanide and cyanogen chloride, chlorine and arsenical gases have all been used separately with more or less success as fumigating agents, germicides, disinfectants and for the extermination of insects and animals over time.
Cyanogen gas (C2N2) has been known as a deadly poison and was recently discovered to be suitable for use as a fumigant (WO 96/01051).
Dichlorvos is a kind of organophosphorus and organochlorine pesticide. Dichlorvos has poor penetration in grains and leaves long-term residues. There are also problems with insects becoming resistant to dichlorvos.
Isothiocyanate esters are generally presented as their crystalline solids. Typically delivery of the isothiocyanate ester fumigants is by sublimation of the solid crystals following from their high vapour pressures. Isothiocyanic esters dissolved in sulphuryl fluoride are also able to be transported. Following evaporation of the sulphuryl fluoride, crystals of the isothiocyanate ester form on the surfaces of structures or commodities. The isothiocyanate ester crystals can then act as a fumigant by sublimation in the usual way.
Other fumigants that have been used against grain pests include acrylonitrile, carbon disulphide, carbon tetrachloride, chloropicrin, ethylene dibromide, ethylene dichloride, ethylene oxide and sulphuryl fluoride.
It will be noted that none of the “conventional” fumigants have ideal fumigant properties and it is phosphine which is set to become the only registered fumigant available for farm use in Australia.
For many years there has been a constant search for new fumigants and there is no doubt that the quest for improved fumigants will continue. There is a particularly urgent requirement for the development of multi-functional grain treatments for on-farm use which should ideally be inexpensive and easy to handle and administer, particularly in unsealed storage containers such as farm bins.